The present invention relates to methods and circuits for selectively enabling one of two filters, and more particularly to a method and circuit for a telephone system in which two filters connected to different inputs share a common capacitor to thereby reduce the number of capacitors needed for the telephone system.
Many systems may include passive filters that have resistors and capacitors, such as low pass and band pass filters. For example, and with reference to FIG. 1, an input 12 may provide a signal to filter 14 having a resistor R1 and a capacitor C1. The input signal is applied between node N1 and ground, and the filtered output appears at node N2. A separate filter 14 has heretofore been required for each input and output pair.
When the system includes an integrated circuit (IC) the resistors in the filters may be integrated into the IC. However, the filters' capacitors, if larger than a few tens of picofarads, are discrete components that are not integrated into the IC Each filter may require one or more such capacitors and thus systems having a plurality of filters each with discrete component capacitors, such as audio frequency circuits in telephone systems, may find that considerable real estate is used by the discrete component capacitors. Further, discrete components may be costly and increase the time needed for assembly.
Some of the systems include a plurality of circuits that are not operated at the same time so that the filter(s) associated with the circuits are not operated at the same time. For example, in a telephone system the ring trip detection circuit (RTDC) and the subscriber line interface circuit (SLIC) are not operated at the same time. The RTDC is connected to a subscriber's telephone line to determine when the telephone has been answered, but is disconnected form the line, and the SLIC is connected to the line, once the telephone has been answered. The SLIC and the RTDC each has a separate low pass filter, and it is desirable to reduce the number of components in the filters.
Accordingly, it is an object of the present invention to provide a novel method and circuit with filters connected to inputs that operate at different times that obviates the problems of the prior art.
It is another object of the present invention to provide a novel method and circuit in which filters connected to inputs that operate at different times share a common capacitor.
It is yet another object of the present invention to provide a novel method and circuit in which two filters share a common capacitor and in which a resistor in one filter and the common capacitor are grounded when the other filter is operating.
It is still another object of the present invention to provide a novel method and circuit in which two filters connected to circuits that do not operate concurrently share a common capacitor and in which a resistor in a filter in the circuit that is not operating is grounded when the other circuit is operating.
It is further object of the present invention to provide a novel method and circuit for operating a telephone system in which a filter in a SLIC and a filter in a RTDC share a common capacitor and in which a resistor in the SLIC filter is grounded to disable the SLIC filter when the RTDC is operating, and in which a resistor in the RTDC filter is grounded to disable the RTDC filter when the SLIC is operating.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.